1. Field of the Invention
This invention relates to a device enclosed within plastic resin. Plastic enclosing is widely used for various semiconductor devices including high voltage devices because they can be enclosed by a simple manufacturing process and such enclosure is suitable for automatic assembling.
2. Description of the Prior Art
A prior device is shown in FIG. 1. A transitor chip 1 is mounted on a first lead 2. The transistor has a breakdown voltage of about 1500V, and high voltage is applied thereto. The first lead 2 includes a mounting portion 3 on which a semiconductor chip is mounted and an external lead portion 4. The transistor chip 1 is bonded on the mounting portion 3 and the collector thereof is electrically connected with the mounting portion 3. The base of the transistor 1 is connected with a second lead 5 through a wire 6 and the emitter of the transistor 1 is connected with a third lead 7 through a wire 8. The end portion of the leads 2, 5 and 7, as well as the chip 1, are enclosed in a plastic enclosure 9, but a back surface of the mounting portion 3 is exposed and a hole 10 is made in the plastic enclosure 9. The hole 10 is used for fixing to an external heat radiation plate with a screw.
The practical application of the device in FIG. 1 is shown in FIG. 2. The device 11 is installed in a printed circuit board 12 and an external heat radiation plate 13. The leads 2, 5 and 7 are secured to the printed board 12 with solder 14 and the body of the device 11 is secured to the external heat radiation plate 13 with a screw 15.
In this application, if high voltage is applied to the device 11 in an atmosphere with moisture, a current leakage passage is apt to be formed between the leads 2 and 5 or the leads 2 and 7, and a current leakage passage may be formed between the leads 2, 5 and 7 and the external heat radiation plate 13. This is ture because the leads are closely arranged and the plastic enclosure 9 between them is flat and has a short surface distance, as is true for the relationship between the leads and the external heat radiation plate. Generally, the higher the applied voltage, the more the current leak is increased jointly with the moisture. When the current leakage passage between the leads is formed, the plastic enclosure 9 is heated by the current leak and the plastic may be carbonized. Carbonized plastic is conductive, so the current leak is further increased. Occasionally, smoke may be emitted and the plastic begins to burn. This may destroy the device.
Moreover, some devices are used in desert conditions. As such, the devices will be subjected to humidity of more than 100% because of the difference of temperatures between the daytime and the night. Also, salt may be included in the moisture. So, the current leakage passage will be more apt to be formed because water including salt is more conductive than pure water.